Monitoring seismic wave velocities in situ

Beginning in the early 1960's, reports from the Soviet Union described travel-time anomalies of 5 to 20 percent preceding large earthquakes. In the early 970's, similar observations began to be reported outside the U.S.S.R. The most convincing were anomalously low values of the velocity ration, V_p/V_s, before four earthquakes of magnitudes 2.5 to 3.3 at Blue Mountain Lake, N.Y.; the anomalies were based on large amounts of high-quality data. In Japan, significant decreases were observed in the travel-time ratio, t_s/t_p, before two thrust-type earthquakes of magnitudes 6. and 5.3. Finally, there is the much discussed report of an anomaly before the magnitude 6.4 San Fernando, Calif., earthquake of 1971 and the implication that the change was caused principally by a decrease in the velocity of the primary (P) wave.